1. Field of the Invention
The invention concerns an electrical signal transmission device protected against electromagnetic interference. It also concerns a method of protecting a cable against electromagnetic interference.
2. Description of the Prior Art
The density of electromagnetic waves transmitted by various means, including by radio, is constantly increasing due to the expansion of telecommunications and the increasing number of radio and television transmitters. This increase in density leads to an increased risk of interference for equipment of all kinds. The commonest example of pollution of this kind is interference on signal transmission cables arising from electromagnetic waves, the cables generally constituting receiving antennas.
Inductor and capacitor filters have been used until now to protect apparatus or equipment connected to the cables. These filters are relatively complicated and costly. The complexity and the cost increase with the bandwidth of the signals to be eliminated.
European patent application 624 885 in the name of Alcatel Cable describes a cable featuring intrinsic filtration of electromagnetic interference in the frequency band below 1 GHz. This coaxial cable comprises a metal core surrounding by at least two layers, one of which is a layer of dielectric material and the other of which, disposed between the core and this layer of dielectric material over at least a portion of the length of the cable, is a layer of a composite semiconductor material comprising an insulative matrix and a non-doped conductive polymer with conjugate bonds. This cable can eliminate the need to use discrete filters. However, the upper limit of 1 GHz is not suited to all applications.
The invention is aimed at providing a signal transmission device combating electromagnetic interference over a wide range of frequencies and that is simple and economic to manufacture. By "electromagnetic interference" is meant radiated interference detected by the cable acting as an antenna. Interference normally transmitted by the cable, that is to say by its core, is not considered here.